Germs live everywhere. You can find germs (microbes) in the air; on food, plants and animals; in soil and water — on just about every other surface, including your body.

Most germs won't harm you. Your immune system protects you against infectious agents. However, some germs are formidable adversaries because they're constantly mutating to breach your immune system's defenses. Knowing how germs work can increase your chances of avoiding infection.

Infectious agents come in a variety of shapes and sizes. Categories include:

Bacteria

Viruses

Fungi

Protozoa

Helminths

Bacteria

Bacteria are one-celled organisms visible only with a microscope. They're so small that if you lined up a thousand of them end to end, they could fit across the end of a pencil eraser.

Not all bacteria are harmful, and some bacteria that live in your body are helpful. For instance, Lactobacillus acidophilus — a harmless bacterium that resides in your intestines — helps you digest food, destroys some disease-causing organisms and provides nutrients.

Many disease-causing bacteria produce toxins — powerful chemicals that damage cells and make you ill. Bacteria cause diseases such as:

Strep throat

Tuberculosis

Urinary tract infections

Viruses

Viruses are much smaller than cells. In fact, viruses are basically just capsules that contain genetic material. To reproduce, viruses invade cells in your body, hijacking the machinery that makes cells work. Host cells are often eventually destroyed during this process.

Viruses are responsible for causing numerous diseases, including:

AIDS

Common cold

Ebola hemorrhagic fever

Genital herpes

Influenza

Measles

Chickenpox and shingles

Antibiotics have no effect on viruses.

Fungi

There are many varieties of fungi, and we eat quite a few of them. Mushrooms are fungi, as is the mold that forms the blue or green veins in some types of cheese. And yeast, another type of fungus, is a necessary ingredient in most types of bread.

Other fungi can cause illness. One example is candida — a yeast that can cause infection. Candida can cause thrush — an infection of the mouth and throat — in infants and in people taking antibiotics or who have an impaired immune system. Fungi are also responsible for skin conditions such as athlete's foot and ringworm.

Protozoa

Protozoa are single-celled organisms that behave like tiny animals — hunting and gathering other microbes for food. Many protozoa call your intestinal tract home and are harmless. Others cause diseases, such as:

Giardia

Malaria

Toxoplasmosis

Protozoa often spend part of their life cycle outside of humans or other hosts, living in food, soil, water or insects. Some protozoa invade your body through the food you eat or the water you drink. Others, such as malaria, are transmitted by mosquitoes.

Helminths

Helminths are among the larger parasites. The word "helminth" comes from the Greek for "worm." If this parasite — or its eggs — enters your body, it takes up residence in your intestinal tract, lungs, liver, skin or brain, where it lives off your body's nutrients. Helminths include tapeworms and roundworms.

There's a difference between infection and disease. Infection, often the first step, occurs when bacteria, viruses or other microbes that cause disease enter your body and begin to multiply. Disease occurs when the cells in your body are damaged — as a result of the infection — and signs and symptoms of an illness appear.

In response to infection, your immune system springs into action. An army of white blood cells, antibodies and other mechanisms goes to work to rid your body of whatever is causing the infection. For instance, in fighting off the common cold, your body might react with fever, coughing and sneezing.

What's the best way to stay disease-free? Prevent infections. You can prevent infection through simple tactics, such as washing your hands regularly, being careful with food and water, getting vaccinations, and taking appropriate medications.

Hand-washing. Often overlooked, hand-washing is one of the easiest and most effective ways to protect yourself from germs and most infections. Wash your hands thoroughly before preparing or eating food, after coughing or sneezing, after changing a diaper, and after using the toilet. When soap and water aren't available, alcohol-based hand-sanitizing gels can offer protection.

Vaccines. Vaccination is your best line of defense for certain diseases. As researchers understand more about what causes disease, the list of vaccine-preventable diseases continues to grow. Many vaccines are given in childhood, but adults still need to be routinely vaccinated to prevent some illnesses, such as tetanus and influenza.

Medicines. Some medicines offer short-term protection from particular germs. For example, taking an anti-parasitic medication might keep you from contracting malaria if you travel to or live in a high-risk area.

Seek medical care if you suspect that you have an infection and you have experienced any of the following:

An animal or human bite

Difficulty breathing

A cough lasting longer than a week

Periods of rapid heartbeat

A rash, especially if it's accompanied by a fever

Swelling

Blurred vision or other difficulty seeing

Persistent vomiting

An unusual or severe headache

Your doctor can perform diagnostic tests to find out if you're infected, the seriousness of the infection and how best to treat that infection.

References

What are microbes? National Institute of Allergy and Infectious Diseases. http://www.niaid.nih.gov/topics/microbes/Pages/default.aspx. Accessed Dec. 16, 2013.

Understanding microbes in sickness and in health. National Institute of Allergy and Infectious Diseases. http://www.niaid.nih.gov/_LAYOUTS/NIAID.Internet.Controls/SearchResults.aspx?getfields=description&q=microbes. Accessed Dec. 16, 2013.

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